I. Field of the Invention
This invention relates to cements used for current collector bar-carbon block joints of electrolytic reduction cells using molten salt electrolytes, e.g. those used for the production of aluminum.
II. Description of the Prior Art
Aluminum is conventionally produced by the reduction of alumina in a "Hall-Heroult" electrolytic cell provided with a lining made of prebaked carbon blocks. The lining acts both as a refractory material to protect the cell walls and bottom from the hot molten electrolyte and aluminum, an as a cathode for the electrolysis process. Current is conveyed from the carbon lining by steel collector bars which extend into slots in the carbon blocks. The slots are made slightly larger than the collector bars to allow for ease of assembly, different rates of expansion of the steel and carbon and slight movements of the collector bars. However, the electrical connection between the carbon lining blocks and the steel collector bars must be good, so an electrically conductive material is generally used to fill the gaps between the bars and the carbon blocks.
When a conductive cement is used in bar-block joints it is traditionally a hot ramming mix consisting of a carbonaceous aggregate, such as calcined anthracite, and a binder, such as pitch or a tar-pitch mixture. The mix is tamped into the joint at a temperature of about 100.degree. C. to 130.degree. C. The use of pitch or tar-pitch mixtures as a binder causes environmental problems. During the filling of the joints, workers are exposed to tar fumes from the hot mix and to noise generated by tamping tools.
Attempts have been made to overcome these problems by providing room temperature cements (normally consisting of a carbonaceous aggregate and a polymeric binder or a low softening point carbonaceous binder) which are tamped or hand pressed into the bar-block joints. While the use of these cements reduces the exposure of workers to fumes, they still require a tamping or pressing step which is difficult and (when tamping tools are used) noisy. Both the problem of fume generation and the need for a tamping or pressing step could be avoided if a room temperature cement of high fluidity could be developed. The cement could then be poured into the bottom of a slot and the collector bar placed over the cement. The weight of the collector bar would then force the fluid cement up around the sides of the bar and thus entirely fill the bar block joint without the need for tamping or pressing. After suitable curing of the cement, the assemblies could then be inverted and the blocks installed in a cell with the slots on the undersides. However, such a cement would not only have to have the correct fluidity requirements, it would also have to perform adequately at cell operating temperatures, and the shrinkage of the cement when exposed to high temperature is of particular importance.
During carbonization, the mixture in the bar-block joint shrinks and the steel bar and carbon slot expand. Expansion of the steel bar is almost four times higher than that of the carbon slot. This is partly compensated for by the shrinkage of the cement in the joint, but if the cement shrinks by too little the block wings may crack, whereas if it shrinks by too much, a gap is formed betwen the bar and the block which results in poor electrical contact. It has been found that a cement having a linear shrinkage in the range of about 4 to 5% can avoid the problems indicated above if used in a gap of 3 to 6 mm between the block and the bar. However, it is not an easy matter to produce a cement having both high fluidity and a linear shrinkage such that it can be used in gaps of 3 to 6 mm. If the particle size of the aggregate is reduced and the binder content is increased to maintain high fluidity, this results in increased shrinkage.
An object of the invention, accordingly, is to provide a room temperature cement which is fluid enough to be used in the manner outlined above but which also has a suitably low linear shrinkage to permit use in bar-block joints of the type described.